Abstract: A film thickness detector system has first and second thickness detector positions. The first position includes a light source for projecting a light beam to a light detector. A guide roller positions a film strip such that the light beam is tangent to a portion of the film strip supported by the guide roller. The film strip lies between the light source and light detector and the light beam is intercepted by a portion of the film strip at an angle with respect to the longitudinal axis of the film strip. As the thickness of the film strip intercepting a portion of the light beam varies, the output from the light detector varies in a corresponding manner. The output is amplified and may be used for stopping motion of the film. The second thickness detector position also has a light source and at least one light detector positioned to receive a light beam from the light source.